DON'T MISS: "Elseworlds" – Superheroes unite! It's time for an epic, three-part mashup of "The Flash," "Arrow" and "Supergirl." The crossover fun – and weirdness – begins when Barry Allen (Grant Gustin) and Oliver Queen (Stephen Amell) wake up one morning and realize they have swapped bodies with each other. They set off to find out what disturbed the timeline to cause such a radical role reversal and eventually realize they need Supergirl's (Melissa Benoist) help. Follow along with "The Flash" (8 p.m. Sunday), "Arrow" (8 p.m., Monday) and "Supergirl" (8 p.m. Tuesday) on The CW.